Can they? Are no other teams going after Nyquist? I don't think it's as simple as you can sign this player instead. What happens if they miss out on Nyquist or some other player they really wanted to fill that gap? Should they say "Oh, well. There's always next year. Let's see what somebody else in our pipeline can do?"
I understand not wanting to give up a first round pick. But Dallas should be in a win-now model with the players they have. Maybe it doesn't happen next year, but being a contender should be their highest priority, not stocking their prospect pool with a potential 20+ first round pick, or bombing out completely and getting a higher pick / lottery pick. They have some older players, but this isn't a team going into a complete rebuild in 2-3 years where they made the mistake of not accumulating prospects sooner rather than later. They still have a solid core of younger players.
Call me bias because I am a NYR fan, and obviously want another first round pick, (I don't even care if it's protected or not), but even trying to look at it without being a NYR fan, Zucc makes a lot of sense to them.
Also, your biggest concern seems to be Zucc is going to delcine over a long term. Sure, depending on the term in might not make sense, but I don't think a 4 year deal is that unreasonable for him to sign. I'm not sure Zucc is looking for this ridiculous long term contract.